Multi-port circulator

ABSTRACT

An extendable four-port circulator includes a middle birefringent crystal, a first birefringent crystal, a first non-reciprocal device, a second birefringent crystal, and a second non-reciprocal device. The first non-reciprocal device is coupled to the first birefringent crystal. The second non-reciprocal device is coupled to the second birefringent crystal. The middle birefringent crystal includes a first surface, a second surface, a third surface, and a fourth surface. The first surface is coupled to the first non-reciprocal device. The second surface is coupled to the second non-reciprocal device. The third surface defines first and second extension interfaces. The fourth surface defines third and fourth extension interfaces. A multi-port circulator includes at least one extendable four-port circulator.

The present invention relates generally to optical technology.

BACKGROUND OF THE INVENTION

A circulator is often used with other optical devices to achieve certainoptical functions. For example, a circulator can be used with a BragGrating to extract an optical signal with a particular wavelength from aWavelength Division Multiplexing (“WDM”) optical signal. FIG. 1 shows afour-port circulator 444 with four ports 1, 2, 3, and 4. An opticalsignal entering port 1 exits from port 2, while an optical signalentering port 2 exits from port 3, and an optical signal entering port 3exits from port 4.

SUMMARY OF THE INVENTION

In one aspect, the invention provides an extendable four-portcirculator. The extendable four-port circulator includes a middlebirefringent crystal, a first birefringent crystal, a firstnon-reciprocal device, a second birefringent crystal, and a secondnon-reciprocal device. The first non-reciprocal device is coupled to thefirst birefringent crystal. The second non-reciprocal device is coupledto the second birefringent crystal. The middle birefringent crystalincludes a first surface, a second surface, a third surface, and afourth surface. The first surface is coupled to the first non-reciprocaldevice. The second surface is coupled to the second non-reciprocaldevice. The third surface defines a first and a second extensioninterface. The fourth surface defines a third and a fourth extensioninterface.

In another aspect, the invention provides a multi-port circulator. Themulti-port circulator includes a middle birefringent crystal, a firstcommon non-reciprocal device, a second common non-reciprocal device, afirst common birefringent crystal, and a second common birefringentcrystal. The first common non-reciprocal device is coupled to the middlebirefringent crystal. The second common non-reciprocal device is coupledto the middle birefringent crystal. The first common birefringentcrystal is coupled to the first common non-reciprocal device. The secondcommon birefringent crystal is coupled to the second commonnon-reciprocal device.

In another aspect, the invention provides a multi-port circulator. Themulti-port circulator includes a middle birefringent crystal, a firstand a second common non-reciprocal device, a first and a third sidebirefringent crystal, and a second and a fourth side birefringentcrystal. The first and the second common non-reciprocal devices each arecoupled to the middle birefringent crystal. The first and the third sidebirefringent crystals each are coupled to the first commonnon-reciprocal device. The second and the fourth side birefringentcrystals each are coupled to the second common non-reciprocal device.

In another aspect, the invention provides a multi-port circulator. Themulti-port circulator includes a middle birefringent crystal, a firstand a third non-reciprocal device, a second and a fourth non-reciprocaldevice, a first side birefringent crystal, a second side birefringentcrystal, a third side birefringent crystal, and a fourth sidebirefringent crystal. The first and the third non-reciprocal deviceseach are coupled to the middle birefringent crystal. The second and thefourth non-reciprocal devices each are coupled to the middlebirefringent crystal. The first side birefringent crystal is coupled tothe first non-reciprocal device. The second side birefringent crystal iscoupled to the second non-reciprocal device. The third side birefringentcrystal is coupled to the third non-reciprocal device. The fourth sidebirefringent crystal is coupled to the fourth non-reciprocal device.

Aspects of the invention can include one or more of the followingadvantages. An extendable four-port circulator in an implementation ofthe instant invention may be cascaded to form a multi-port circulator.Other advantages will be readily apparent from the attached figures andthe description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a four-port circulator.

FIG. 2a illustrates a twelve-port circulator having ports 1001-1012.

FIG. 2b illustrates an extendable four-port circulator having four portsand four extension interfaces.

FIG. 2c illustrates three extendable four-port circulators cascadedtogether to form a twelve-port circulator.

FIG. 2d illustrates the position and orientation of the components of anextendable four port PM circulator shown in FIG. 2c.

FIG. 3a(1)-FIG. 3a(3) illustrate that an optical signal introduced atport 1 is separated into two light beams that are recombined to exitfrom port 2.

FIG. 3b(1)-FIG. 3b(3) illustrate that an optical signal introduced atport 2 is separated into two light beams that are recombined to exitfrom port 3.

FIG. 3c(1)-FIG. 3c(3) illustrate that an optical signal introduced atport 3 is separated into two light beams that are recombined to exitfrom port 4.

FIG. 3d(1)-FIG. 3d(3) illustrate that an optical signal introduced atport 4 is separated into two light beams that exit from two of theextension interfaces.

FIG. 3e(1)-FIG. 3e(3) illustrate that light beams entering two of theextension interfaces are combined to exit from port 1.

FIG. 4a-FIG. 4e summarize the optical paths in the y-z plane traveled bythe light beams respectively in FIG. 3a-FIG. 3e.

FIG. 4f illustrates the optical paths in the x-z plane traveled by thelight beams in FIG. 4a-FIG. 4e

FIG. 5 illustrates three extendable four-port circulators of FIG. 2cascaded together to form a twelve-port circulator.

FIG. 6 and FIG. 7 illustrate alternative implementations of thetwelve-port circulator of FIG. 5.

FIG. 8a and FIG. 8b illustrate an implementation of non-reciprocaldevice 120 having one half wave plate and two Faraday rotators.

FIG. 9a and FIG. 9b illustrate an implementation of non-reciprocaldevice 120 having two half wave plates and one Faraday rotator.

FIG. 10a and FIG. 10b illustrate an implementation of non-reciprocaldevice 160 having one half wave plate and two Faraday rotators.

FIG. 11a and FIG. 11b illustrate an implementation of non-reciprocaldevice 160 having two half wave plates and one Faraday rotator.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an improvement in optical technology.The following description is presented to enable one of ordinary skillin the art to make and use the invention and is provided in the contextof a patent application and its requirements. Various modifications tothe invention will be readily apparent to those skilled in the art andthe generic principles herein may be applied to other embodiments. Thus,the present invention is not intended to be limited to the embodimentsshown, but is to be accorded the widest scope consistent with theprinciples and features described herein.

The present invention will be described in terms of an extendablefour-port circulator and a twelve-port circulator having specificcomponents having specific configurations. Similarly, the presentinvention will be described in terms of components having specificrelationships, such as distances or angles between components. However,one of ordinary skill in the art will readily recognize that this methodand system will operate effectively for other components having similarproperties, other configurations, and other relationships betweencomponents. In the instant application, the implementations of anextendable four-port circulator are described. Two extendable four-portcirculators can be cascaded to form an eight-port circulator, and threeextendable four-port circulators can be cascaded to form a twelve-portcirculator.

FIG. 2a shows a twelve-port circulator 1000 that includes twelve ports1001-1012. As shown in FIG. 2a, an optical signal entering port 1001will exit from port 1002, and an optical signal entering port 1002 willexit from port 1003. Similarly, an optical signal entering port 1003,1004, 1005, 1006, 1007, 1008, 1009, 1010, and 1011 will exit,respectively, from port 1004, 1005, 1006, 1007, 1008, 1009, 1010, 1011,and 1012.

Twelve-port circulator 1000 can be constructed with a number ofdifferent methods. One possible implementation of a twelve-portcirculator is to cascade three extendable four-port circulators. Anextendable four-port circulator 555 described in the instant applicationis shown in FIG. 2b. Extendable four-port circulator 555 includes fourports 1, 2, 3, and 4, along with extension interfaces 0 a, 0 b, 5 a, and5 b. An optical signal entering port 1 will exit from port 2, an opticalsignal entering port 2 will exit from port 3, and an optical signalentering port 3 will exit from port 4. In addition, an optical signalsentering extension interfaces 0 a and 0 b will be merged and exit fromport 1, and an optical signal entering port 4 will be split into twooptical signals exiting respectively from ports 5 a and 5 b.

FIG. 2c shows that three extendable four-port circulators 555′, 555, and555″ are cascaded together to form a twelve-port circulator 1000.Interfaces 5 a′ and 5 b′ of circulator 555′ are respectively coupled tointerfaces 0 a and 0 b of circulator 555, and interfaces 5 a and 5 b ofcirculator 555 are respectively coupled to interfaces 0 a″ and 0 b″ ofcirculator 555″. Ports 1′, 2′, 3′, and 4′ of four-port circulator 555′are respectively equivalent to ports 1001, 1002, 1003, and 1004 oftwelve-port circulator 1000; ports 1, 2, 3, and 4 of four-portcirculator 555 are respectively equivalent to ports 1005, 1006, 1007,and 1008 of twelve-port circulator 1000; and ports 1″, 2″, 3″, and 4″ offour-port circulator 555″ are respectively equivalent to ports 1009,1010, 1011, and 1012 of twelve-port circulator 1000.

FIG. 2d illustrates an implementation of an extendable four-portcirculator 555.

Circulator 555 includes dual fiber collimator 100, birefringent crystal110, non-reciprocal device 120, wedge 130, birefringent crystal 140,wedge 150, non-reciprocal device 160, birefringent crystal 170, and dualfiber collimator 200. Each of dual fiber collimators 100 and 200 can becoupled to two fibers (not shown). Circulator 555 includes four ports 1,2, 3, and 4, along with extension interfaces 0 a, 0 b, 5 a, and 5 b. Twocouplings at collimator 100 constitute respectively ports 1 and 3, andtwo couplings at collimator 200 constitute respectively ports 2 and 4.Two areas on surface 142 of birefringent crystal 140 constituteextension interfaces 0 a and 0 b, and two areas on surface 144 ofbirefringent crystal 140 constitute extension interfaces 5 a and 5 b.

A light beam may enter one of four regions of a given component inextendable four-port circulator 555. The four regions are labeled asquadrant I, II, III, and IV, as shown in FIG. 2d. The x-direction,y-direction and the z-direction are also shown in the figure. Thepositive z-direction is along the propagation direction of a light beamintroduced at dual fiber collimator 100.

FIGS. 3a(1), 3 a(2), and 3 a(3) illustrate that an optical signalintroduced at port 1 is separated into light beams 12 a and 12 b. Lightbeams 12 a and 12 b are recombined and exit from port 2. FIG. 3a(1) is aperspective view, FIG. 3a(2) is a planar view on the y-z plane, and FIG.3a(3) is a planar view on the x-z plane.

The optical signal introduced at port 1 is separated into light beam 12a with the y-polarization and light beam 12 b with the x-polarization inbirefringent crystal 110. Light beam 12 a is not deflected and exitsfrom quadrant II of birefringent crystal 110. Light beam 12 b isdeflected in the positive x-direction and exits from quadrant I ofbirefringent crystal 110. After exiting from birefringent crystal 110,light beams 12 a and 12 b travel in the positive z-direction andsomewhat in the positive y-direction.

Light beam 12 a enters quadrant II of non-reciprocal device 120 with they-polarization, and exits from quadrant II with the x-polarization.After passing through quadrant II of wedge 130, light beam 12 a isdeflected to travel essentially in alignment with the positivez-direction. Light beam 12 a then passes through quadrant II ofbirefringent crystal 140, without being deflected. After passing throughquadrant II of wedge 150, light beam 12 a is deflected to travel in thepositive z-direction and somewhat in the negative y-direction (FIG.3a(2)). Thereafter, light beam 12 a enters quadrant II of non-reciprocaldevice 160 with the x-polarization and exits from quadrant II with thex-polarization. Finally, light beam 12 a enters quadrant II ofbirefringent crystal 170, is deflected in the positive x-direction, andenters dual fiber collimator 200 with the x-polarization.

Light beam 12 b enters quadrant I of non-reciprocal device 120 with thex-polarization, and exits from quadrant I with the x-polarization. Afterpassing through quadrant I of wedge 130, light beam 12 b is deflected totravel essentially in alignment with the positive z-direction. Lightbeam 12 b then passes through quadrant I of birefringent crystal 140,without being deflected. After passing through quadrant I of wedge 150,light beam 12 b is deflected to travel in the positive z-direction andsomewhat in the negative y-direction (FIG. 3a(2)). Thereafter, lightbeam 12 b enters quadrant I of non-reciprocal device 160 with thex-polarization, and exits from quadrant I with the y-polarization.Finally, light beam 12 b passes through quadrant I of birefringentcrystal 170, without being deflected, and enters dual fiber collimator200 with the y-polarization.

Light beams 12 a and 12 b enter collimator 200 respectively with thex-polarization and the y-polarization, are combined at collimator 200,and exit from port 2.

FIGS. 3b(1), 3 b(2), and 3 b(3) illustrate that an optical signalintroduced at port 2 is separated into light beams 23 a and 23 b. Lightbeams 23 a and 23 b are recombined and exit from port 3. FIG. 3b(1) is aperspective view, FIG. 3b(2) is a planar view on the y-z plane, and FIG.3b(3) is a planar view on the x-z plane.

The optical signal introduced at port 2 is separated into light beam 23a with the x-polarization and light beam 23 b with the y-polarization.Light beam 23 a is deflected in the negative x-direction and exits fromquadrant II of birefringent crystal 170. Light beam 23 b is notdeflected and exits from quadrant I of birefringent crystal 170. Afterexiting from birefringent crystal 170, light beams 23 a and 23 b travelin the negative z-direction and somewhat in the positive y-direction(FIG. 3b(2)).

Light beam 23 a enters quadrant II of non-reciprocal device 160 with thex-polarization, and exits from quadrant II with the y-polarization.After passing through quadrant II of wedge 150, light beam 23 a isdeflected to travel essentially in alignment with the negativez-direction. Light beam 23 a then enters quadrant II of birefringentcrystal 140, is deflected in the negative y-direction, and exits fromquadrant III of birefringent crystal 140. After passing through quadrantIII of wedge 130, light beam 23 a is deflected to travel in the negativez-direction and somewhat in the positive y-direction. Thereafter, lightbeam 23 a enters quadrant III of non-reciprocal device 120 with they-polarization, and exits from quadrant III with the y-polarization.Finally, light beam 23 a passes through quadrant III of birefringentcrystal 110, without being deflected, and enters dual fiber collimator100 with the y-polarization.

Light beam 23 b enters quadrant I of non-reciprocal device 160 with they-polarization, and exits from quadrant I with the y-polarization. Afterpassing through quadrant I of wedge 150, light beam 23 b is deflected totravel essentially in alignment with the negative z-direction. Lightbeam 23 a then enters quadrant I of birefringent crystal 140, isdeflected in the negative y-direction, and exits from quadrant IV ofbirefringent crystal 140. After passing through quadrant IV of wedge150, light beam 23 b is deflected to travel in the negative z-directionbut leaning toward the positive y-direction. Thereafter, light beam 23 benters quadrant IV of non-reciprocal device 120 with the y-polarization,and exits the x-polarization. Finally, light beam 23 b enters quadrantIV of birefringent crystal 110, is deflected in the negativex-direction, and enters dual fiber collimator 100 with thex-polarization.

Light beams 23 a and 23 b enter collimator 100 respectively with they-polarization and the x-polarization, are combined at collimator 100,and exit from port 3.

FIGS. 3c(1), 3 c(2), and 3 c(3) illustrate that an optical signalintroduced at port 3 is separated into light beams 34 a and 34 b. Lightbeams 34 a and 34 b are recombined and exit from port 4. FIG. 3c(1) is aperspective view, FIG. 3c(2) is a planar view on the y-z plane, and FIG.3c(3) is a planar view on the x-z plane.

The optical signal introduced at port 3 is separated into light beam 34a with the y-polarization and light beam 34 b with the x polarization bybirefringent crystal 110. Light beam 34 a is not deflected and exitsfrom quadrant III of birefringent crystal 110. Light beam 34 b isdeflected in the positive x-direction and exits from quadrant IV ofbirefringent crystal 110. After exiting from birefringent crystal 110,light beams 34 a and 34 b travel in the positive z-direction andsomewhat in the negative y-direction (FIG. 3c(2)).

Light beam 34 a enters quadrant III of non-reciprocal device 120 withthe y-polarization, and exits from quadrant III with the x-polarization.After passing through quadrant III of wedge 130, light beam 34 a isdeflected to travel essentially in alignment with the positivez-direction. Light beam 34 a then passes through quadrant III ofbirefringent crystal 140, without being deflected. After passing throughquadrant III of wedge 150, light beam 34 a is deflected to travel in thepositive z-direction and somewhat in the positive y-direction.Thereafter, light beam 34 a enters quadrant III of non-reciprocal device160 with the x-polarization, and exits from quadrant III with thex-polarization. Finally, light beam 34 a enters quadrant II ofbirefringent crystal 170, is deflected in the positive x-direction, andenters dual fiber collimator 200 with the x-polarization.

Light beam 34 b enters quadrant IV of non-reciprocal device 120 with thex-polarization, and exits from quadrant I with the x-polarization. Afterpassing through quadrant IV of wedge 130, light beam 34 b is deflectedto travel essentially in alignment with the positive z-direction. Lightbeam 34 b then passes through quadrant IV of birefringent crystal 140,without being deflected. After passing through quadrant IV of wedge 150,light beam 34 b is deflected to travel in the positive z-direction andsomewhat in the positive y-direction (FIG. 3c(2)). Thereafter, lightbeam 34 b enters quadrant IV of non-reciprocal device 160 with thex-polarization, and exits from quadrant IV with the y-polarization.Finally, light beam 34 b passes through quadrant IV of birefringentcrystal 170, and enters dual fiber collimator 200 with they-polarization.

Light beams 34 a and 34 b enter collimator 200 respectively with thex-polarization and the y-polarization, are combined at collimator 200,and exit from port 4.

FIGS. 3d(1), 3 d(2), and 3 d(3) illustrate that an optical signalintroduced at port 4 is separated into light beams 45 a and 45 b. Lightbeams 45 a and 45 b exit respectively from extension interfaces 5 a and5 b. FIG. 3d(1) is a perspective view, FIG. 3d(2) is a planar view onthe y-z plane, and FIG. 3d(3) is a planar view on the x-z plane.

The optical signal introduced at port 4 on dual fiber collimator 200 isseparated into light beam 45 a with the x-polarization and light beam 45b with the y-polarization by birefringent crystal 170. Light beam 45 ais deflected in the negative x-direction and exits from quadrant III ofbirefringent crystal 170. Light beam 45 b is not deflected and exitsfrom quadrant IV of birefringent crystal 170. After exiting frombirefringent crystal 170, light beams 45 a and 45 b travel in thenegative z-direction and somewhat in the negative y-direction (FIG.3d(2)).

Light beam 45 a enters quadrant III of non-reciprocal device 160 withthe x-polarization, and exits from quadrant III with the y-polarization.After passing through quadrant III of wedge 150, light beam 45 a isdeflected to travel essentially in alignment with the negativez-direction. Light beam 45 a then enters quadrant III of birefringentcrystal 140, is deflected in the negative y-direction, and exits withthe y-polarization from extension interface 5 a on surface 144 ofbirefringent crystal 140.

Light beam 45 b enters quadrant IV of non-reciprocal device 160 with they-polarization, and exits from quadrant IV with the y-polarization.After passing through quadrant IV of wedge 150, light beam 45 b isdeflected to travel essentially in alignment with the negativez-direction. Light beam 45 b then enters quadrant IV of birefringentcrystal 140, is deflected in the negative y-direction, and exits fromextension interface 5 b on surface 144 of birefringent crystal 140 withthe y-polarization.

FIGS. 3e(1), 3 e(2), and 3 e(3) illustrate that light beams 01 a and 01b enter respectively extension interfaces 0 a and 0 b of extendablefour-ports circulator 555, are combined, and exit from port 1. FIG.3e(1) is a perspective view, FIG. 3e(2) is a planar view on the y-zplane, and FIG. 3e(3) is a planar view on the x-z plane.

Light beam 01 a enters extension interface 0 a on surface 142 ofbirefringent crystal 140 with the y-polarization, and exits fromquadrant II of birefringent crystal 140 in a direction that is inalignment with the negative z-direction. After passing through quadrantII of wedge 130, light beam 01 a is deflected to travel in a directionthat resembles the negative z-direction but leaning toward the negativey-direction (FIG. 3e(2)). Thereafter, light beam 01 a enters quadrant IIof non-reciprocal device 120 with the y-polarization, and exits fromquadrant II with the y-polarization. Finally, light beam 01 b passesthrough quadrant I of birefringent crystal 110, without being deflected,and enters dual fiber collimator 100 with the y-polarization.

Light beam 01 b enters extension interface 0 b on surface 142 ofbirefringent crystal 140 with the y-polarization, and exits fromquadrant I of birefringent crystal 140 in the negative z-direction.After passing through quadrant I of wedge 130, light beam 01 b isdeflected to travel in the negative z-direction and somewhat in thenegative y-direction (FIG. 3e(2)). Thereafter, light beam 01 b entersquadrant I of non-reciprocal device 120 with the y-polarization, andexits from quadrant I with the x-polarization. Finally, light beam 01 benters quadrant I of birefringent crystal 110, is deflected in thenegative x-direction, and enters dual fiber collimator 100 with thex-polarization.

Light beams 01 a and 01 b enter collimator 100 respectively with they-polarization and the x-polarization, are combined at collimator 100,and exit from port 1.

FIGS. 4a, 4 b, 4 c, 4 d, and 4 e, shown in the y-z plane, show theoptical paths traveled by the light beams respectively in FIGS. 3a(3), 3b(3), 3 c(3), 3 d(3), and 3 e(3). FIG. 4f shows the optical paths in thex-z plane. FIGS. 4a-4 e show the paths traveled by light beamsintroduced at ports 1, 2, 3, and 4, and at extension interfaces 0 a and0 u, respectively. In each figure, the actual paths traveled by lightbeams are represented by arrow lines.

FIG. 4a shows that an optical signal introduced at port 1 is separatedinto light beams 12 a and 12 b, and light beams 12 a and 12 b arerecombined to exit from port 2. FIG. 4b shows that an optical signalintroduced at port 2 is separated into light beams 23 a and 23 b, andlight beams 23 a and 23 b are recombined to exit from port 3. FIG. 4cshows that an optical signal introduced at port 3 is separated intolight beams 34 a and 34 b, and light beams 34 a and 34 b are recombinedto exit from port 4. FIG. 4d shows that an optical signal introduced atport 4 is separated into light beams 45 a and 45 b, and light beams 45 aand 45 b exit respectively from extension interfaces 5 a and 5 b. FIG.4e shows that light beams 01 a and 01 b enter respectively extensioninterfaces 0 a and 0 b, and are combined to exit from port 1.

FIG. 5 shows three extendable four-port circulators 555′, 555, and 555″cascaded together to form twelve-port circulator 1000. Extensioninterfaces 5 a′ and 5 b′ of circulator 555′ (not shown) are respectivelycoupled to extension interfaces 0 a and 0 b of circulator 555 (notshown) by directly contacting surface 144′ of circulator 555′ withsurface 142 of circulator 555. In one implementation, extensioninterfaces 5 a and 5 b of circulator 555 (not shown) are respectivelycoupled to extension interfaces 0 a″ and 0 b″ of circulator 555″ (notshown) by directly contacting surface 144 of circulator 555 with surface142″ of circulator 555″. Extension interfaces 0 a′ and 0 b′ on surface142′ (not shown) of circulator 555′ and extension interfaces 5 a″ and 5b″ on surface 144″ (not shown) of circulator 555″ are not used andtherefore need not to be implemented. Birefringent crystal 140′, 140,and 144″ can be replaced with a single birefringent crystal 410.

FIG. 6 shows an alternative implementation of twelve-port circulator1000. Twelve-port circulator 1000 includes four pair of reflectors:reflectors 311 and 312, reflectors 321 and 322, reflectors 511 and 512,and reflectors 521 and 522. Optical paths traveling in dual fibercollimator 100′, birefringent crystal 110′, non-reciprocal device 120′,and wedge 130′ are shifted together in the positive y-direction usingreflectors 311 and 312. Optical paths traveling in wedge 150′,non-reciprocal device 160′, birefringent crystal 170′, and dual fibercollimator 200 are shifted together in the positive y-direction usingreflectors 321 and 322. Optical paths traveling in dual fiber collimator100″, birefringent crystal 110″, non-reciprocal device 120″, and wedge130″ are shifted together in the negative y-direction using reflectors511 and 512. Finally, Optical paths traveling in wedge 150″,non-reciprocal device 160″, birefringent crystal 170″, and dual fibercollimator 200″ are shifted together in the negative y-direction usingreflectors 521 and 522.

Alternative implementations of twelve-port circulator 1000 can also useone pair, two pair, or three pairs of reflectors, instead of four pairs.Further, a single reflector can be used to replace a pair of reflectors.For example, if only reflector 312 is used and reflector 311 iseliminated, it is possible to rotate together by 90 degrees theorientations of dual fiber collimator 100′, birefringent crystal 110′,non-reciprocal device 120′, and wedge 130′, such that the propagationdirection of a light beam introduced at dual fiber collimator 100 isinitially in the negative y-direction. In the implementations oftwelve-port circulator 1000, reflectors, pairs of reflectors, or wedgesmay be generally referred to as path-conditioning components.

FIG. 7 shows another implementation of twelve-port circulator 1000including at least one common component for replacing a group ofindividual components in FIG. 5. For example, birefringent crystals110′, 110, and 110″ in FIG. 5 can be replaced with common birefringentcrystal 101 in FIG. 7. Similarly, birefringent crystals 170′, 170, and170″ can be replaced with common birefringent crystal 710.Non-reciprocal devices 120′, 120, and 120″ can be replaced with commonnon-reciprocal device 210. Non-reciprocal devices 160′, 160, and 160″can be replaced with common non-reciprocal device 610.

As described above, the functions of each component in extendablefour-port circulator 555 (FIG. 2) may depend on both the direction andthe quadrant that a light beam enters. The construction of eachcomponent in extendable four-port circulator 555 of FIG. 2 is describedbelow. The functions of each component, as a light beam travels in thepositive z-direction, are described with respect to FIGS. 3a and 3 c.Likewise, the function of each component, as a light beam travels in thenegative z-direction, are described with respect to FIGS. 3b, 3 d, and 3e.

Birefringent crystal 110 is constructed and orientated in such a way toperform the following functions: (1) light passing through birefringentcrystal 110 in the positive z-direction with the y-polarization will notbe deflected, and light with the x-polarization will be deflected in thepositive x-direction; (2) light passing through birefringent crystal 110in the negative z-direction with the y-polarization will not bedeflected, and light beam with the x-polarization will be deflected inthe negative x-direction. Accordingly, birefringent crystal 110 splitsor joins light beams in accordance with their respective polarizations.The polarization of the o-ray in birefringent crystal 110 is in they-direction.

Non-reciprocal device 120 is constructed to perform the followingfunctions: (1) light passing through non-reciprocal device 120 in thepositive z-direction and entering device 120 through quadrant I or IVwith the x-polarization remains as light with the x-polarization, andlight entering device 120 through quadrant II or III with they-polarization becomes light with the x-polarization; (2) light passingthrough non-reciprocal device 120 in the negative z-direction andentering device 120 through quadrant I or IV with the y-polarizationwill become light with the x-polarization, and light entering device 120through quadrant II or III with the y-polarization remains as light withthe y-polarization.

Birefringent crystal 140 is constructed and orientated in such a way toperform the following functions: (1) light passing through birefringentcrystal 140 in the positive z-direction with the x-polarization will notbe deflected; (2) light passing through birefringent crystal 140 in thenegative z-direction with the y-polarization will be deflected in thenegative y-direction. The polarization of the o-ray in birefringentcrystal 140 is in the x-direction.

Non-reciprocal device 160 is constructed to perform the followingfunctions: (1) light passing through non-reciprocal device 160 in thepositive z-direction and entering device 160 through quadrant I or IVwith the x-polarization will become light with the y-polarization, andlight entering device 160 through quadrant II or III with thex-polarization remains as light with the x-polarization; (2) lightpassing through non-reciprocal device 160 in the negative z-directionand entering device 160 through quadrant I or IV with the y-polarizationremain as light with the y-polarization, and light entering device 160through quadrant II or III with the x-polarization will become lightwith the y-polarization.

One implementation of non-reciprocal device 120, as shown in FIGS. 8aand 8 b, includes half wave plate 122 and Faraday rotators 125 a and 125b. In one implementation, the optical axis of half wave plate 122 is inthe direction of a vector rotated +22.5 degrees from the positivex-direction. When a light beam passes through Faraday rotator 125 a, ineither the positive or the negative z-directions, the polarization ofthe light beam will be rotated by +45 degrees with respect to thepositive z-axis. When a light beam passes through Faraday rotator 125 b,either in the positive or the negative z-directions, the polarization ofthe light beam will be rotated by −45 degrees with respect to thepositive z-axis.

As shown in FIG. 8a, after passing through half wave plate 122 in thepositive z-direction, a light beam with the x-polarization becomes alight beam with the x+y polarization, and a light beam with they-polarization becomes a light beam with the x-y polarization. Afterpassing through Faraday rotator 125 a, a light beam with the x-ypolarization, is rotated +45 degrees, and becomes a light beam with thex-polarization. Likewise, after passing through Faraday rotator 125 b, alight beam with the x+y polarization, is rotated −45 degrees, andbecomes a light beam with the x-polarization.

As shown in FIG. 8b, after passing through Faraday rotator 125 a in thenegative z-direction, a light beam with the y-polarization, is rotated+45 degrees, and becomes a light beam with the x-y polarization. A lightbeam with the x-y polarization, after passing through half wave plate122, becomes a light beam with the y-polarization. After passing throughFaraday rotator 125 b in the negative z-direction, a light beam with they-polarization, is rotated −45 degrees, and becomes a light beam withthe x+y polarization. A light beam with the x+y polarization, afterpassing through half wave plate 122, becomes a light beam with thex-polarization.

In the implementations of FIGS. 8a and 8 b, when the position of halfwave plate 122 is exchanged with the position of Faraday rotators 125 aand 125 b, the functions of non-reciprocal device 120 remain the same.It is also possible to choose other optical axes for half wave plate 122and other rotation directions for Faraday rotators 125 a and 125 b.

Another implementations of non-reciprocal device 120, as shown in FIGS.9a and 9 b, include half wave plates 122 a and 122 b and Faraday rotator125. The optical axis of half wave plate 122 a is in the direction of avector rotated +22.5 degrees from the positive x-direction. The opticalaxis of half wave plate 122 b is in the direction of a vector rotated−22.5 degrees from the positive x-direction. When a light beam passesthrough Faraday rotator 125, in either the positive or the negativez-directions, the polarization of the light beam will be rotated by +45degrees.

As shown in FIG. 9a, a light beam with the y-polarization, after passingthrough half wave plate 122 a in the positive z-direction becomes alight beam with the x+y polarization. Likewise, a light beam with thex-polarization, after passing through half wave plate 122 b in thepositive z-direction, becomes a light beam with the x-y polarization.The light beams with the x-y polarization, after passing through Faradayrotators 125, are rotated +45 degrees, become light beams with thex-polarization.

As shown in FIG. 9b, a light beam with the y-polarization, after passingthrough Faraday rotators 125 in the negative z-direction, is rotated +45degrees and becomes light beams with the x-y polarization. A light beamwith the x-y polarization, after passing through half wave plate 122 a,becomes a light beam with the y-polarization. A light beam with the x-ypolarization, after passing through half wave plate 122 b, becomes alight beam with the y-polarization.

In the implementation of FIGS. 9a and 9 b, when the position of halfwave plates 122 a and 122 b is exchanged with the position of Faradayrotators 125, the functions of non-reciprocal device 120 remain thesame. Other optical axes for half wave plate 122 a and 122 b and otherrotation directions for Faraday rotator 125 can be selected.

Similar to non-reciprocal device 120, common non-reciprocal device 210(in FIG. 7) can be constructed using one half wave plate in combinationwith two Faraday rotators, or using two half wave plates in combinationwith one Faraday rotator.

Non-reciprocal device 160 can be implemented in a number of differentways. One implementation of non-reciprocal device 160, as shown in FIGS.10a and 10 b, includes half wave plate 162 and Faraday rotators 165 aand 165 b. The optical axis of half wave plate 162 is in the directionof a vector rotated +22.5 degrees from the positive x-direction. When alight beam passes Faraday rotators 165 a, in either the positive or thenegative z-directions, the polarization of the light beam will berotated by −45 degrees. When a light beam passes Faraday rotators 165 b,either in the positive or the negative z-directions, the polarization ofthe light beam will be rotated by +45 degrees.

As shown in FIG. 10a, light beams with the x-polarization, after passingthrough half wave plate 162 in the positive z-direction, become lightbeams with the x+y polarization. A light beam with the x+y polarization,after passing through Faraday rotator 165 a, is rotated −45 degrees, andbecomes a light beam with the x-polarization. A light beam with the x+ypolarization, after passing through Faraday rotator 165 b, is rotated+45 degrees, and becomes a light beam with the y-polarization.

As shown in FIG. 10b, after passing through Faraday rotators 165 a inthe negative z-direction, a light beam with the x-polarization, rotated−45 degrees, become a light beam with the x-y polarization. Likewise,after passing through Faraday rotators 165 b in the negativez-direction, a light beam with the y-polarization, rotated +45 degrees,become a light beam with the x-y polarization. The light beams with thex-y polarization, after passing through half wave plate 162, becomelight beams with the y-polarization.

In the implementation of FIGS. 10a and 10 b, when the position of halfwave plate 162 is exchanged with the position of Faraday rotators 165 aand 165 b, the functions of non-reciprocal device 160 remain the same.Other optical axes for half wave plate 162 and other rotation directionsfor Faraday rotators 165 a and 165 b.

Another implementation of non-reciprocal device 160, as shown in FIGS.11a and FIG. 11b, includes half wave plates 162 a and 162 b and Faradayrotator 165. The optical axis of half wave plate 162 a is in thedirection of a vector rotated −22.5 degrees from the positivex-direction. The optical axis of half wave plate 162 b is in thedirection of a vector rotated +22.5 degrees from the positivex-direction. When a light beam passes through Faraday rotators 165, ineither the positive or the negative z-directions, the polarization ofthe light beam will be rotated by +45 degrees.

As shown in FIG. 11a, a light beam with the x-polarization, afterpassing through half wave plate 162 a in the positive z-direction,becomes a light beam with the x-y polarization. A light beam with thex-polarization, after passing through half wave plate 162 b in thepositive z-direction, becomes a light beam with the x+y polarization.After passing through Faraday rotator 165 and being rotated +45 degrees,a light beam with the x-y polarization becomes a light beam with thex-polarization, and a light beam with the x+y polarization becomes alight beam with the y-polarization.

As shown in FIG. 11b, after passing through Faraday rotator 165 in thenegative z-direction and being rotated +45 degrees, a light beam withthe x-polarization becomes a light beam with the x+y polarization, and alight beam with the y-polarization becomes a light beam with the x-ypolarization. After passing through half wave plate 162 a, a light beamwith the x+y polarization becomes a light beam with the y-polarization.Likewise, after passing through half wave plate 162 b, a light beam withthe x-y polarization becomes a light beam with the y-polarization.

In the implementation of FIGS. 11a and 11 b, the position of half waveplates 162 a and 162 b can be exchanged with the position of Faradayrotator 165, and the functions of non-reciprocal device 160 remainunchanged. Other optical axes for half wave plate 162 a and 162 b andother rotation directions for Faraday rotator 165 can be selected.

Similar to non-reciprocal device 160, common non-reciprocal device 610(in FIG. 7) can be constructed using one half wave plate in combinationwith two Faraday rotators, or using two half wave plates in combinationwith one Faraday rotator.

A method and system has been disclosed for providing extendablefour-port circulators, which may be cascaded or combined to form atwelve-port circulator. Although the present invention has beendescribed in accordance with the embodiments shown, one of ordinaryskill in the art will readily recognize that there could be variationsto the embodiments and those variations would be within the spirit andscope of the present invention. For example, two extendable four-portcirculators can be cascaded or combined to form an eight-portcirculator, and four can be cascaded or combined to form a sixteen-portcirculator. In general, an integer number N of extendable four-portcirculators can be cascaded or combined to form a 4N-port circulator.Accordingly, many modifications may be made by one of ordinary skill inthe art without departing from the spirit and scope of the appendedclaims.

What is claimed is:
 1. An extendable four-port circulator comprising: amiddle birefringent crystal; a first birefringent crystal; a firstnon-reciprocal device coupled to the first birefringent crystal; asecond birefringent crystal; a second non-reciprocal device coupled tothe second birefringent crystal; and wherein the middle birefringentcrystal having a first surface coupled to the first non-reciprocaldevice, a second surface coupled to the second non-reciprocal device, athird surface defining a first and a second extension interfaces, and afourth surface defining a third and a fourth extension interfaces. 2.The extendable four-port circulator of claim 1 further comprising afirst dual fiber collimator coupled to the first birefringent crystaland adapted to be coupled to a first fiber and a third fiber; a seconddual fiber collimator coupled to the second birefringent crystal andadapted to be coupled to a second fiber and a fourth fiber.
 3. Theextendable four-port circulator of claim 2 further comprising a firstwedge placed at a position selected from the group consisting of: aposition between the first dual fiber collimator and the firstnon-reciprocal device, and a position between the first non-reciprocaldevice and the middle birefringent crystal.
 4. The extendable four-portcirculator of claim 3 further comprising a second wedge placed at aposition selected from the group consisting of: a position between thesecond dual fiber collimator and the second non-reciprocal device, and aposition between the second non-reciprocal device and the middlebirefringent crystal.
 5. The extendable four-port circulator of claim 1,wherein the first non-reciprocal device includes two half wave platescoupled to a Faraday rotator.
 6. The extendable four-port circulator ofclaim 1, wherein the first non-reciprocal device includes a half waveplate coupled to two Faraday rotators.
 7. The extendable four-portcirculator of claim 1, wherein the second non-reciprocal device includestwo half wave plates coupled to a Faraday rotator.
 8. The extendablefour-port circulator of claim 1, wherein the second non-reciprocaldevice includes a half wave plate coupled to two Faraday rotators. 9.The extendable four-port circulator of claim 1, wherein the polarizationof the o-ray in the first birefringent crystal is substantiallyorthogonal to the polarization of the o-ray in the middle birefringentcrystal.
 10. The extendable four-port circulator of claim 1, wherein thepolarization of the o-ray in the second birefringent crystal issubstantially orthogonal to the polarization of the o-ray in the middlebirefringent crystal.
 11. A multi-port circulator comprising: a middlebirefringent crystal; a first common non-reciprocal device coupled tothe middle birefringent crystal; a second common non-reciprocal devicecoupled to the middle birefringent crystal; a first common birefringentcrystal coupled to the first common non-reciprocal device; and a secondcommon birefringent crystal coupled to the second common non-reciprocaldevice.
 12. The multi-port circulator of claim 11 further comprising afirst dual fiber collimator adapted to be coupled to a first fiber and athird fiber; a second dual fiber collimator adapted to be coupled to asecond fiber and a fourth fiber; a third dual fiber collimator adaptedto be coupled to a fifth fiber and a seventh fiber; a fourth dual fibercollimator adapted to be coupled to a sixth fiber and an eighth fiber;and wherein the first common birefringent crystal being coupled to thefirst and the third dual fiber collimator, and the second commonbirefringent crystal being coupled to the second and the fourth dualfiber collimator.
 13. The multi-port circulator of claim 11, wherein atleast one of the common non-reciprocal devices includes two half waveplates coupled to a Faraday rotator.
 14. The multi-port circulator ofclaim 11, wherein at least one of the common non-reciprocal devicesincludes a half wave plate coupled to two Faraday rotators.
 15. Themulti-port circulator of claim 11, wherein the polarization of the o-rayin each of common birefringent crystals is substantially orthogonal tothe polarization of the o-ray in the middle birefringent crystal. 16.The multi-port circulator of claim 11 further comprising a wedge placedat a position selected from the group consisting of a position betweenthe first common non-reciprocal device and the middle birefringentcrystal; a position between the second common non-reciprocal device andthe middle birefringent crystal, a position between the first commonbirefringent crystal and the first common non-reciprocal device, and aposition between the second common birefringent crystal and the secondcommon non-reciprocal device.
 17. The multi-port circulator of claim 12further comprising a wedge placed at a position selected from the groupconsisting of a position between the first common non-reciprocal deviceand the middle birefringent crystal; a position between the secondcommon non-reciprocal device and the middle birefringent crystal, aposition between the first common birefringent crystal and the firstcommon non-reciprocal device, a position between the second commonbirefringent crystal and the second common non-reciprocal device, aposition between the first dual fiber collimator and the first commonbirefringent crystal, a position between the second dual fibercollimator and the second common birefringent crystal, a positionbetween the third dual fiber collimator and the first commonnon-reciprocal device, and a position between the fourth dual fibercollimator and the second common non-reciprocal device.
 18. Themulti-port circulator of claim 11 further comprising a pair ofreflectors placed at a position selected from the group consisting of aposition between the first common non-reciprocal device and the middlebirefringent crystal; a position between the second commonnon-reciprocal device and the middle birefringent crystal, a positionbetween the first common birefringent crystal and the first commonnon-reciprocal device, and a position between the second commonbirefringent crystal and the second common non-reciprocal device. 19.The multi-port circulator of claim 12 further comprising a pair ofreflectors placed at a position selected from the group consisting of aposition between the first common non-reciprocal device and the middlebirefringent crystal; a position between the second commonnon-reciprocal device and the middle birefringent crystal, a positionbetween the first common birefringent crystal and the first commonnon-reciprocal device, a position between the second common birefringentcrystal and the second common non-reciprocal device, a position betweenthe first dual fiber collimator and the first common birefringentcrystal, a position between the second dual fiber collimator and thesecond common birefringent crystal, a position between the third dualfiber collimator and the first common non-reciprocal device, and aposition between the fourth dual fiber collimator and the second commonnon-reciprocal device.
 20. The multi-port circulator of claim 11 furthercomprising a reflector placed at a position selected from the groupconsisting of a position between the first dual fiber collimator and thefirst common birefringent crystal, a position between the second dualfiber collimator and the second common birefringent crystal, a positionbetween the third dual fiber collimator and the first commonnon-reciprocal device, and a position between the fourth dual fibercollimator and the second common non-reciprocal device.
 21. A multi-portcirculator of claim 11 further comprising: a fifth dual fiber collimatorcoupled to the first common birefringent crystal and adapted to becoupled to a ninth fiber and an eleventh fiber; and a sixth dual fibercollimator coupled to the second common birefringent crystal and adaptedto be coupled to a tenth fiber and a twelfth fiber.
 22. The multi-portcirculator of claim 20, wherein at least one of the commonnon-reciprocal devices includes two half wave plates coupled to aFaraday rotator.
 23. The multi-port circulator of claim 20, wherein atleast one of the common non-reciprocal devices includes a half waveplate coupled to two Faraday rotators.
 24. The multi-port circulator ofclaim 20, wherein the polarization of the o-ray in each of commonbirefringent crystals is substantially orthogonal to the polarization ofthe o-ray in the middle birefringent crystal.
 25. A multi-portcirculator comprising: a middle birefringent crystal; a first and asecond common non-reciprocal device each coupled to the middlebirefringent crystal; a first and a third side birefringent crystal eachcoupled to the first common non-reciprocal device; and a second and afourth side birefringent crystal each coupled to the second commonnon-reciprocal device.
 26. The multi-port circulator of claim 25 furthercomprising a path-conditioning component placed at a position selectedfrom the group consisting of a position between the first sidebirefringent crystal and the first common non-reciprocal device, aposition between the second side birefringent crystal and the secondcommon non-reciprocal device, a position between the third sidebirefringent crystal and the first common non-reciprocal device, and aposition between the fourth side birefringent crystal and the secondcommon non-reciprocal device.
 27. The multi-port circulator of claim 26wherein the path-conditioning component is a wedge.
 28. The multi-portcirculator of claim 26 wherein the path-conditioning component is a pairof reflectors.
 29. The multi-port circulator of claim 26 wherein thepath-conditioning component is a reflector.
 30. The multi-portcirculator of claim 25 further comprising a first dual fiber collimatorcoupled to the first side birefringent crystal and adapted to be coupledto a first fiber and a third fiber; a second dual fiber collimatorcoupled to second side birefringent crystal and adapted to be coupled toa second fiber and a fourth fiber; a third dual fiber collimator coupledto the third side birefringent crystal and adapted to be coupled to afifth fiber and a seventh fiber; and a fourth dual fiber collimatorcoupled to the fourth side birefringent crystal and adapted to becoupled to a sixth fiber and an eighth fiber.
 31. The multi-portcirculator of claim 30 further comprising a path-conditioning componentplaced at a position selected from the group consisting of a positionbetween the first dual fiber collimator and the first side birefringentcrystal, a position between the first side birefringent crystal and thefirst common non-reciprocal device, a position between the second dualfiber collimator and the second side birefringent crystal, a positionbetween the second side birefringent crystal and the second commonnon-reciprocal device, a position between the third dual fibercollimator and the third side birefringent crystal, a position betweenthe third side birefringent crystal and the first common non-reciprocaldevice, a position between the fourth dual fiber collimator and thefourth side birefringent crystal, and a position between the fourth sidebirefringent crystal and the second common non-reciprocal device. 32.The multi-port circulator of claim 31 wherein the path-conditioningcomponent is a wedge.
 33. The multi-port circulator of claim 31 whereinthe path-conditioning component is a pair of reflectors.
 34. Themulti-port circulator of claim 31 wherein the path-conditioningcomponent is a reflector.
 35. The multi-port circulator of claim 25,wherein at least one of the common non-reciprocal devices includes twohalf wave plates coupled to a Faraday rotator.
 36. The multi-portcirculator of claim 25, wherein at least one of the commonnon-reciprocal devices includes a half wave plate coupled to two Faradayrotators.
 37. The multi-port circulator of claim 25, wherein thepolarization of the o-ray in each of the side birefringent crystals issubstantially orthogonal to the polarization of the o-ray in the middlebirefringent crystal.
 38. The multi-port circulator of claim 25 furthercomprising a wedge placed at a position between one of the commonnon-reciprocal devices and the middle birefringent crystal.
 39. Themulti-port circulator of claim 25 further comprising a pair ofreflectors placed at a position between one of the common non-reciprocaldevices and the middle birefringent crystal.
 40. A multi-port circulatorof claim 25 further comprising: a fifth side birefringent crystalcoupled to the first common non-reciprocal device; and a sixth sidebirefringent crystal coupled to the second common non-reciprocal device.41. The multi-port circulator of claim 40 further comprising: a fifthdual fiber collimator coupled to the fifth side birefringent crystal andadapted to be coupled to a ninth fiber and an eleventh fiber; a sixthdual fiber collimator coupled to the sixth side birefringent crystal andadapted to be coupled to a tenth fiber and a twelve fiber.
 42. Themulti-port circulator of claim 41, wherein at least one of the commonnon-reciprocal devices includes two half wave plates coupled to aFaraday rotator.
 43. The multi-port circulator of claim 41, wherein atleast one of the common non-reciprocal devices includes a half waveplate coupled to two Faraday rotators.
 44. The multi-port circulator ofclaim 41, wherein the polarization of the o-ray in each of the sidebirefringent crystals is substantially orthogonal to the polarization ofthe o-ray in the middle bireftingent crystal.
 45. A multi-portcirculator comprising: a middle birefringent crystal; a first and athird non-reciprocal device each coupled to the middle birefringentcrystal; a second and a fourth non-reciprocal device each coupled to themiddle birefringent crystal; a first side birefringent crystal coupledto the first non-reciprocal device; a second side birefringent crystalcoupled to the second non-reciprocal device; a third side birefringentcrystal coupled to the third non-reciprocal device; and a fourth sidebirefringent crystal coupled to the fourth non-reciprocal device. 46.The multi-port circulator of claim 45 further comprising: a first dualfiber collimator coupled to the first side birefringent crystal andadapted to be coupled to a first fiber and a third fiber; a second dualfiber collimator coupled to second side birefringent crystal and adaptedto be coupled to a second fiber and a fourth fiber; a third dual fibercollimator coupled to the third side birefringent crystal and adapted tobe coupled to a fifth fiber and a seventh fiber; and a fourth dual fibercollimator coupled to the fourth side birefringent crystal and adaptedto be coupled to a sixth fiber and an eighth fiber.
 47. The multi-portcirculator of claim 45, wherein at least one of the commonnon-reciprocal devices includes two half wave plates coupled to aFaraday rotator.
 48. The multi-port circulator of claim 45, wherein atleast one of the common non-reciprocal devices includes a half waveplate coupled to two Faraday rotators.
 49. The multi-port circulator ofclaim 45, wherein the polarization of the o-ray in each of the sidebirefringent crystals is substantially orthogonal to the polarization ofthe o-ray in the middle birefringent crystal.
 50. The multi-portcirculator of claim 45 further comprising a path-conditioning componentplaced at a position selected from the group consisting of a positionbetween the first side birefringent crystal and the first non-reciprocaldevice, a position between the first non-reciprocal device and themiddle birefringent crystal, a position between the second sidebirefringent crystal and the second non-reciprocal device, a positionbetween the second non-reciprocal device and the middle birefringentcrystal, a position between the third side birefringent crystal and thethird non-reciprocal device, a position between the third non-reciprocaldevice and the middle birefringent crystal, a position between thefourth side birefringent crystal and the fourth non-reciprocal device,and a position between the fourth non-reciprocal device and the middlebirefringent crystal.
 51. The multi-port circulator of claim 46 furthercomprising a path-conditioning component placed at a position selectedfrom the group consisting of a position between the first dual fibercollimator and the first side birefringent crystal, a position betweenthe first side birefringent crystal and the first non-reciprocal device,a position between the first non-reciprocal device and the middlebirefringent crystal, a position between the second dual fibercollimator and the second side birefringent crystal, a position betweenthe second side birefringent crystal and the second non-reciprocaldevice, a position between the second non-reciprocal device and themiddle birefringent crystal, a position between the third dual fibercollimator and the third side birefringent crystal, a position betweenthe third side birefringent crystal and the third non-reciprocal device,a position between the third non-reciprocal device and the middlebirefringent crystal, a position between the fourth dual fibercollimator and the fourth side birefringent crystal, a position betweenthe fourth side birefringent crystal and the fourth non-reciprocaldevice, and a position between the fourth non-reciprocal device and themiddle birefringent crystal.
 52. The multi-port circulator of claim 51wherein the path-conditioning component is a wedge.
 53. The multi-portcirculator of claim 51 wherein the path-conditioning-component is a pairof reflectors.
 54. The multi-port circulator of claim 51 wherein thepath-conditioning component is a reflector.
 55. The multi-portcirculator of claim 45 further comprising: a fifth non-reciprocal devicecoupled to the middle birefringent crystal; a fifth side birefringentcrystal coupled to the fifth non-reciprocal device; a fifth dual fibercollimator coupled to the fifth side birefringent crystal and adapted tobe coupled to a ninth fiber and an eleventh fiber; a sixthnon-reciprocal device coupled to the middle birefringent crystal; asixth side birefringent crystal coupled to the sixth commonnon-reciprocal device; and a sixth dual fiber collimator coupled to thesixth side birefringent crystal and adapted to be coupled to a tenthfiber and a twelfth fiber.
 56. The multi-port circulator of claim 55,wherein at least one of the common non-reciprocal devices includes twohalf wave plates coupled to a Faraday rotator.
 57. The multi-portcirculator of claim 55, wherein at least one of the commonnon-reciprocal devices includes a half wave plate coupled to two Faradayrotators.
 58. The multi-port circulator of claim 55, wherein thepolarization of the o-ray in each of the side birefringent crystals issubstantially orthogonal to the polarization of the o-ray in the middlebirefringent crystal.